Process and furnace for heat application

ABSTRACT

Control of speed and uniformity of the heating of the working zone of the preform in a glass drawing operation in which the softening heat is applied by forced convection (13), using a separate fluid heating zone to introduce temperature and velocity controlled fluid. The size of the working zone is further controlled by changing (18) the configuration of a movable exit sleeve. Uniformity and accuracy of temperature is enhanced by normalization (10) of the temperature of the preform close to a preselected value before the preform enters the furnace. 
     A glass drawing apparatus embodies the preferred means to carry out the process, providing a two-chamber furnace having an air heating chamber (44) connected by air delivery channels (51) to a drawing chamber (43). The drawing chamber has preform inlet (41) and a drawn product outlet (56). Each of the latter preferably has an adjustable opening diameter. The inlet is provided with a pre-cooling collar (35). The outlet has an insulated, movable sleeve (62) controlling the effective distance from the inlet to the outlet by modifying the temperature gradient. The latter provides means for controlling the length and shape of the working zone (55).

This application is a continuation of application Ser. No. 798,585,filed Nov. 15, 1985, now abandoned.

TECHNICAL FIELD

This invention is directed to the working of softenable dielectricmaterials, particularly the drawing of clad and unclad fibers and fiberbundles from primary and later-stage preforms of glass. The inventionparticularly relates to processes for drawing glass fibers, bundles, andcomposite products from the fused or softened end of a preformintroduced into a furnace.

BACKGROUND ART

The art of glass drawing is presently the most effective mode ofproducing either continuous, flexible fibers or of producing relativelyshort segments for later combining and processing into compositeproducts such as fiberoptic screens, faceplates, and image modifiers ofvarious types. Besides being used for drawing of fibers and multi-fiberbundles, drawing techniques of the type to which the invention relatesare applied to late-stage processing of the composite products. Suchprocessing includes cross-sectional reduction, either uniform orgraduated, the latter technique used to form image expanders andreducers. Such processing also includes various degrees of twisting andother manipulations to form image re-orienting devices such as partialrotators, inverters, etc.

An important goal in this technical field is uniformity of heating and ahigh degree of temperature control in the critical softened area of thepreform or workpiece. Failure of uniformity in heating the work zone isa major cause of product defect and rejection, resulting in waste ofexpensive materials and production time. This consideration isparticularly critical in the case of a product formed from a preform ofhighly complex cross-sectional character in which large, sometimessharp, gradients of optical, physical, and thermodynamic properties arelikely to be present. The requirement of uniform heating reachesultimate criticality when the conventional upper limits of heating anddrawing speed and of preform and product cross-sectional dimension arereached and exceeded. It has been the unrealized goal of skilled workersin this art to produce uniform heating in the drawing furnace at themoderate temperatures needed for drawing relatively delicate compositeproducts. Among the main reasons for failure to achieve this goal hasbeen the difficulty of achieving uniform radiation of heat from theradiant heating elements at temperatures of around 1100° F. to 1400° F.(600° to 750° C.) Separate radiant elements produce inherentlynon-uniform heating. Attempts to produce a radiant source continuouslysurrounding the fusing area or to embed discrete elements in a diffusingmatrix have not produced the desired uniformity. Moreover, mostcomposite products do not absorb radiant energy uniformly even if it isintroduced uniformly. This compounds the problem of non-uniform radiantsources, and limits the level of uniformity even for an ideal uniformradiant source.

This problem of absorbtion differential exists in any application wherethere are different glasses in the same product, the glasses havingdifferent infra-red absorbing characteristics (for instance the corerelative to the cladding) or any product involving an extremely thickpreform or drawn diameter. In the latter case, the rate of heating byabsorption at the surface of the working area must be carefullyregulated according to the rate of conduction of the heat toward thecenter of the piece. At locations toward the axis the radiant energy perse fails to penetrate at levels comparable to that at the surface.

A prior method of approximating uniformly radiating elements has beendeveloped which involves turning the preform and the product on theircommon axis, at a rate sufficient to smooth out the variations in theradiational heating sources. This technique requires complex mechanismsto coordinate the turning of the preform and product, as well as theturning and lateral translation of the take-up reel if the productrequires. At best, the technique produces horizontal (stratified)uniformity without producing vertical uniformity and results in hot"rings" instead of hot "spots". Moreover, the technique does not addressthe problem of non-uniform absorption by a composite product.

The problem of non-uniform absorption is especially acute when theproduct contains light-absorbing elements such as EMA cladding or fiberswhich tend to absorb infra-red radiation in disproportion to theremaining materials. Such elements, in a radiant furnace, produceinternal anomalies of temperature and viscosity which limit andcomplicate the choice of drawing speed.

As a secondary consequence to the inability to achieve uniform heatingin the drawing process, both the preform size and the reduction ratio inthe drawing process are severely limited. The result is that a compositeproduct having very small diameter fiber-optic components must beproduced by a many-step process. Typically, the steps include drawing asingle fiber, drawing down a multi-fiber bundle, drawing a multi-multifiber bundle, and fusing a bundle of these latter products into a block.Such many-stage processes consume production time, and each step has itsown percentage rejection rate (on the order of 20%).

Thus, the main object of the present invention is to provide a processfor acting on the working area of a preform to product highlycontrolled, uniform heating.

Another object of the invention is to provide a process in which thelimits on the size of the preform, the product, and the drawingreduction ratio are greatly extended.

A further object of the invention is to provide a process for drawingglass which allows the elimination of at least one of the successivereduction drawings in certain fiber-optic processes, without loss ofquality.

Another object of the invention is to provide a process in which thesize of the working zone at which reduction takes place may be chosenand controlled.

In another of its aspects, the invention provides and apparatusparticularly adapted to assist in carrying out the uniform heating andcontrol of the working zone of a drawable preform.

Further objects of the invention will become apparent as specificembodiments are described.

DISCLOSURE OF THE INVENTION

The present invention uses a controlled, high-velocity flow oftemperature-regulated air or other fluid, preferably produced in aseparate heating chamber and introduced into a the drawing chamber. Theprocess takes advantage of the temperature distributing qualities of amix of forced-and free-convection to uniformly heat the working zone ofa preform. The process involves removing heat-depleted fluid, preferablyby cycling the cooled fluid back to the separate heating chamber forreheating. The process involves using data from temperature sensors atcritical points in the flow cycle to control fluid heating to maintain adesired smooth temperature/time profile.

In another aspect, the process involves temperature regulation of asmall part of the preform just outside of the drawing chamber of thefurnace in order to insure a consistently temperature-prepared preformentering the working area; this reduces the requirement to regulateambient temperature and to compensate for different thermal conductivityof different preforms and preform clamping means. In effect, this stepforms a thermal insulator which keeps the heat from the furnace fromundesirable transfer up the preform.

A further aspect of the process of the invention involves controlledmovement of an extendable insulation means in order to regulate theeffective distance from the inlet to the outlet of the drawing chamberof the furnace, whereby the size of the working zone is regulated atwill.

A still further aspect of the invention is a drawing furnacespecifically adapted to carry out the process. The furnace comprises adrawing chamber with a preform inlet and outlet and a preferablyseparate fluid heating chamber having controllable heating means. Theseparate fluid heating chamber communicates with the drawing chamber byinput passageways or channels, the communication mediated by forcedconvection means. In accordance with other aspects of the invention, thefurnace is provided with a return channel communicating between thedrawing chamber and the heating chamber, a pre-entry temperatureregulating means at the preform inlet, and a movable insulated sleeveassociated with the outlet of the drawing chamber.

In a variation of both the process and the apparatus, the preform inletis at the bottom of the furnace, and the product is drawn upwardly froman outlet on the top of the furnace drawing chamber. The steps andelements of the variations may be independently employed.

The disclosed process and apparatus solve the technical problem ofuniformly heating large-diameter drawable workpieces, possibly havingcomplex shape in all dimensions and which, in addition, may be acomposite of materials having different radiant absorbingcharacteristics. The high-velocity, forced-convection supply of fluid atthe work zone allows transfer of heat energy to the work without theextreme differences in temperature encountered in radiant heating. Thus,the avoidance of surface hot spots or internal hot spots does not dependexclusively on the conduction rate into the workpiece or on smallamounts of free convection of dead air, but can be controlled byregulation of the velocity and temperature of the forced convectioncurrents. At a given rate of heat energy application, moreover, thetechnical problem of controlling the size of the work zone (which isrelated to the shape of the draw and the shape of the diameter reductionprofile,) may be readily resolved by extension of an insulating sleeveinto the forced convection flow pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

Both the process and the apparatus of the present invention may be bestunderstood by reference to specific embodiments as shown in thedrawings, which are illustrative and not limiting.

FIG. 1 shows a flow diagram of an embodiment of the process of thepresent invention,

FIG. 2 is a sectional view of a furnace embodying the principles of thepresent invention,

FIG. 3 illustrates a simple embodiment of part of a pre-cooling collar,

FIG. 4 is a top elevational view of a preferred form of the drawingchamber inlet with adjustable diameter means,

FIG. 5 is a sectional view on the line V--V of FIG. 2 showing anembodiment of the heated fluid distribution means,

FIG. 6 illustrates the action of the adjustable insulated sleeve, and

FIG. 7 shows a modified apparatus designed to be capable of carrying outthe drawing operation upward instead of downward.

DETAILED DESCRIPTION

There are several modes of carrying out the process of the presentinvention in which some of the details depend upon the product beingmade and the raw materials being worked. The following is a detaileddescription of successful applications, including the best modecontemplated at present. A detailed description of an apparatusspecifically designed for carrying out the process is also laid outhere.

The process diagram shown in FIG. 1 outlines the basic steps in thegeneral process. Since the process allows very close control of thepreform heating, based on calculated and experiential air temperatureand flow-rate data, optimal use of the process first requires a certaindegree of pre-entry temperature control of the preform. This pre-entrytemperature conventionally depends not only on ambient temperature(control of which is inefficient) but also on the conduction rate fromprior furnaces through the preform and its feeding mechanisms. Theeffect of these factors is reduced in the process of the presentinvention by the step of bringing the temperature of the preform closeto a "normalized" temperature just before it enters the furnace. Thiswill most often be a cooling step, although at some stages in theprocess and for some ambient conditions there may be mild warming. Thesimplest embodiment of this temperature normalizing step 10 involvesbringing more or less constant temperature air from a source and blowingit onto the periphery of the preform at the entry point to the furnace.At the same time, the preform is fed into the furnace drawing chamber ina feeding step 11 using available feeding mechanisms. These includemotor driven drive screws. The mechanism may include several of thesedrive screws if the core and a cladding or a plurality of claddings mustbe driven at different rates. It is not necessary for these mechanismsto include rotating means. Such means were a complicating expedient toachieve furnace uniformity as mentioned above and have had onlyqualified success.

The crux of the present invention involves the step of heating air orother heat exchange fluid in a chamber, preferable separate from thechamber in which the fiber drawing will take place. If the chamber isnot separate, at least the working zone should be shielded from anyradiant elements used therein. The heated air is then introduced ordelivered 13 into the drawing chamber using controllable forcedconvection means: fans, air pumps, etc. The air heating step 12 ispreferably controlled (for instance by variable resistance or byintroducing a controlled cool gas stream) under the guidance of atemperature measurement step 14. This measurement is preferably carriedout as the air is introduced into the drawing chamber. This measurement,which is carried out for example by inserting a thermcouple into the airflow pattern, controls the air heating means by a preselected algorithm,mediated by electrical or electronic processing means. These may involvefeedback or feedforward processing with calculated or tabulatedparameters, leading to a discrete or continuous heat control settingstep 15. This step may also encompass control of variation of the flowrate of the heated air. The goal is the development of atemperature/time profile appropriate for the uniform heating of a givenpreform in a given working zone. To this end, the heated air is directedto flow past the working zone of the preform, step 16. The preferredmode of flow is rather turbulent, but with a directional trend throughthe working zone in a distinct direction. It is also preferable that theinflow be distributed around the periphery of the preform inlet end ofthe drawing chamber.

When the preform is at working temperature, the reduced diameter productis drawn in the usual manner, using combinations of gravity and tractionin a drawing step 17. Since the rate of drawing and the degree ofreduction in diameter of preform product depend (among other factors) onboth temperature profile and the length of the working zone, anadditional working zone shaping step 18 may be employed. The workingzone shape may be set before the drawing begins or may be adjusted atvarious stages of the drawing process according to the requirements ofthe product and/or the character of the preform.

A successful method of shaping the working zone involves varying theeffective distance from the inlet of the drawing chamber to theeffective outlet. Specifically, this may be accomplished by positioningan insulating sleeve which is capable of variable extension from theoutlet of the chamber toward the inlet, concentrically of the axis ofthe draw.

To facilitate temperature control, the heated air which has been flowedpast the working zone may be withdrawn from the chamber in an air returnstep 19 to be reheated in the heating chamber. If an additional airtemperature measurement step 20 is performed on withdrawal of the air,refinement of the heat setting control may be made. Such an adjustmentmay be made, for example, on the basis of calculation of absorbed heatfrom the temperature differential.

After drawing of the product, the usual product processing is carriedout, indicated generally as step 21. For continuous fiber, a rotatingand reciprocating take-up reel may be provided. Additional coatings maybe applied. For more discrete product, means may be provided forperiodic cutoff of draw segments, either as quasi-finished product orfor bundling into multi- and multi/multi-element composites for furtherprocessing.

Because this process allows the drawing of very large diameter productfrom very large diameter preforms due to the extraodinarily uniformheating that the process provides, it has been found to be possible andadvantageous to feed the preform up from below the furnace and draw theproduct up from above. This process variation advantageously modifiesthe effect of gravity on the working zone shape and on the take-upqualities of the product. The variation is especially applicable in aworking zone which is relatively long with a slow taper.

The general process of this invention has been applied without anyextraordinary control measures to a composite preform over 4.5 inches(12 cm) in diameter which was drawn down to a diameter of 0.030 inches(0.080 cm) in one stage. The preform was normalized to about 70° F. (21°C.). Air was heated so that it could be delivered to the drawing chamberat about 1380° F. (750° C.). The air was forced at a high velocity pastthe working zone and withdrawn at about 1375° F. (747° C.). The rate ofproduction and the quality of the product were at least comparable toconventional radiative drawing which would have required multiple stagesfor this reduction.

In another case the uniformity of heating in this process allowed aone-step production of a one-inch product from a three-inch preform withexcellent quality and production rate. The preform was introduced frombelow the furnace and taken up from above.

No apparent limit has been found to the size of preform to which theprocess may advantageously be applied. When applied to conventionaldrawing processes, the production rate approaches 5 to 6 times the usualrate and is presently limited by the capacity of available or readilymodified take-up mechanisms.

The ability of this system to deliver heat effectively to large diameterworkpieces in the normal drawing operation without distortion in theworkpiece allows use of the system in a remarkable manner. Ordinarily,large diameter products formed of large numbers of fine fibers areformed by a very inefficient process involving stacking of the fibersand the fusing of the fibers under heat and very high pressure to removevoids. It has been found that these large diameter products can beformed far more efficiently in the following manner. First, a preform isformed by stacking fibers into a bundle with a diameter slightly largerthan the desired product. The bundle is then enclosed in a gas-tightglass envelope which is then evacuated. The resulting preform is thenpassed through the furnace in the manner of this invention except thatthe preform is only drawn down a small amount to the desired diameter.The result is a large diameter product formed of uniformly fused,voidless and undistorted fibers. This product is capable of being slicedinto gas-tight plates. In practical operation, it may be necessary tocontinuously evacuate the envelope during the draw. Because of the verylow distortion caused by this draw, it is possible to apply a twistingmotion to the workpiece in the draw zone. This results in a product inwhich the fibers have a uniform spiral orientation. The resultingproduct can be cut to form image rotators or inverters.

APPARATUS

An apparatus specifically designed to carry out the process of thepresent invention is disclosed in FIGS. 2-7 and is shown mostcomprehensively in a modified schematic manner in FIG. 2.

Although forced convection ovens have been available in the past, thefurnace of the present invention is disclosed in unique combination withother elements and in configuration adapted to utilize the specialproperties of forced convection in a glass drawing operation. Theapplication of the forced convection heating to the glass drawing arthave resulted in extraordinary, unexpected, and surprising increase inthe diameter capacity, drawing rate and quality (in terms of productrejection rates) of this art.

The general combination as illustrated in FIG. 2 and indicated generallyby the numeral 30, includes a complex of feed mechanisms 31, preferablyhaving a capacity for differential feeding of various components of acomposite preform. The feeding complex may include a vacuum pullassembly 32 including the required gas tight seals, and clamping means33. The workpiece of the apparatus is a heat softenable, drawablepreform 34. To carry out the temperature normalization or "pre-cooling"of the preform just before entry into the furnace (indicated generallyby 40) this embodiment includes a hollow collar 35 or thermal isolatorwhich is supplied with relatively constant temperature air from a source36 via a conduit 37 with controllable valve 38. The "pre-cooling" air isdirected inwardly toward the preform through a plurality of inwardlyfacing, radially directed apertures such as 39. It has been successfuland convenient to choose, as a standard, a temperature near roomtemperature of 70° F. (about 21° C.). To achieve a finer degree ofcontrol over keeping the preform temperature constant, temperaturesensors above and/or below the collar could be provided to controlpre-cooling air temperature and/or volume.

The preform 34 is fed into the furnace 40 through a drawing chamberinlet 41. This inlet is preferably supplied with diameter adjustingmeans 42 such as a fairly refractory "iris" assembly, to insure moderateresistance to heat loss.

The drawing chamber 43 is one of two chambers which this embodiment ofthe furnace comprehends, the other chamber being the separate airheating chamber 44. The heating chamber is supplied with heating meanswhich may be combustive, inductive, arc-induced, dielectric, etc., butin the preferred embodiment comprised large area resistive coils 45.These are supplied with power from a source 46 and a control mechanism47 such as variable impedance or a variable transformer.

A forced convection element 50 (a high temperature fan, pump, etc.)draws air through the heating elements and directs a flow through achannel 51 which communicates with the drawing chamber 43. This deliveryis preferably mediated by a distribution means 52 such as an internally,radially perforated plenum.

At a point along the path of flow, a temperature sensing transducermeans 54 is provided sending its signal to a central control system 60.

The flow of air is actively or passively drawn past thepreform/workpiece to create the working zone 55 terminating at or nearthe effective drawing chamber outlet 56. The flow continues through areturn channel 57 which may be provided with a return forced convectionelement 58 and a return air temperature sensing transducer means 59 alsosending to the central control 60.

The effective drawing chamber outlet may be provided with diameteradjusting means 59 similar to the means 42 at the inlet.

The effective drawing chamber outlet 56 is distinguished from the actualoutlet 61 by an insulating sleeve 62 which movably engages the actualoutlet and extends into the drawing chamber toward the inlet 41. Whenthe product is drawn past the inner end of this sleeve, the product isshielded from the heated flow. This movable point thus defines the endof the working zone 55. The position of the sleeve may be temporarilyfixed as by set screws or clamps, or may be under variable centralcontrol effected by extension means 63 such as servo-activated rack andgear or friction wheels.

The product is drawn in the usual manner by a drawing mechanism 64 whichemploys gravity, traction means, etc. The product is then passed on tofurther processing elements 65: take-up reels, cutters, bundlers,slicers, etc.

An effective pre-cooling collar 35 may be constructed as detailed inFIG. 3. Compressed air of relatively constant temperature is brought viaa conduit 37 from a constant temperature air source such as an aircompressor into the collar and is directed inward toward the preformthrough radial apertures 39.

The detailed view in FIG. 4 looking down on the drawing chamber inletillustrates an embodiment of an adjustable diameter means 42 for causingthe effective inlet diameter to approximate that of the preform 34(shown as a typical single cladding fiber-optic preform.) A similarmechanism can be used for the diameter adjusting mechanism 59 of theeffective outlet 56.

FIG. 5 shows a detailed horizontal section on the line V--V of FIG. 2 ofa flow distribution means 52 embodied in a toroidal plenum with internalradial perforations 53 shaped to facilitate flow control. In thepreferred embodiment, the perforations would be elongated along the axisof the workpiece.

A detail of a dynamic embodiment of the adjustable sleeve 62 is shown inFIG. 6 with an adjustment for a shortening of the working zone in brokenlines. Adjustment is made by extending means 63 under central control60. The extending means are preferably annular.

The process variation which involves feeding the preform from below thefurnace and drawing the product from above may be carried outsatisfactorily on the apparatus of FIG. 2. Full advantage of thisinvention may best be taken under the circumstances, however, by theapparatus variation shown in FIG. 7. In this variation, the furnace isso mounted, as by the use of gimbles 70, 71, that it may be rotated atwill in a vertical plane. In this case, the feed mechanisms 31 and thedrawing mechanism 64 are relocated to their respective appropriatelocations as shown in the figure.

As an alternative to the rotatable configuration shown in FIG. 7, theapparatus may be built with a symmetry about a central horizontal plane.Thus, the input channel from the heating chamber to the drawing chamber,as well as the flow distribution means, may be located centrally. The"inlet" and "outlet" areas of the drawing chamber may then beidentically supplied with closable return channels, extendable insulatedsleeves, pre-cooling collars, and aperture diameter adjusting means. Inthis way the preform may be fed and the product drawn upward or downwardwith equal ease. The choice will depend upon the character of preform,product, and production rate.

INDUSTRIAL APPLICABILITY

Some of the modes of industrial applicability of the uniform heatingprovided by the drawing process of the present invention are readilyapparent from the above description of the equipment and itscharacteristics, while other uses and advantages are totally unexpected.

A typical product rejection rate of about 20% at each step of amulti-step manufacturing process obtains in present radiation furnaces.The process and apparatus of the present invention have significantlyreduced this rejection rate.

The drawing rate for moderate diameter products has been greatly limitedby the need for a slow enough rate to allow complete and reasonablyuniform heating of the preform. The present process heats even largediameter preforms quickly and uniformly to the point that some productcan be taken up at rates 5-6 times to that of conventionally configureddrawing furnaces, with more than satisfactory quality.

The present process does not generally require special treatment ofproducts which incorporate radiation absorbing claddings and fibers ascompared to with the treatment required of such products in radiativefurnaces. Such products include most composite image manipulatingproducts: faceplates, image expanders, inverters, etc.

Most significantly, in prior art processes, the normal composite productis produced in several stages. This is due to limitations in the size ofthe reduction ratio of the product to preform which can be achievedwithout distortion. There has also been an absolute size limit of thepreform that can be heated with threshhold uniformity. The presentprocess can uniformly heat the working zone of a preform, including acomposite preform, of at least up to 4.5 inches (12 cm) and possiblymuch larger. This means:

1. At fairly ordinary product-to-preform ratios, a final product oflarge diameter may be drawn, possibly eliminating the final pressing,annealing, devoiding, and shaping steps,

2. Advantage can be taken of the vastly larger reduction ratios possibleto eliminate one or more stages of the production of multi-multi-typecomposite products. The elimination of such steps saves time and labor,as well as the cost of materials lost through inevitable waste andrejection.

Clearly, minor changes may be made in the form and construction of thisinvention and in the embodiments of the process without departing fromthe material spirit of either. Therefore, it is not desired to confinethe invention to the exact forms shown herein and described but it isdesired to include all subject matter that properly comes within thescope claimed.

The invention having been thus described, what is claimed as new anddesired to be secured by Letters Patent is:
 1. A process for drawing afiber or fiber-bundle product from a preform of heat-softenable,drawable material having a drawing temperature and a working zone,comprising the steps of:(a) feeding the preform into an inlet in adrawing chamber, (b) heating a gaseous fluid to a temperature at orabove the drawing temperature of the preform using heating means whichare substantially completely radiatively shielded from the preformworking zone, (c) causing the heated fluid to flow past the preformworking zone until the preform is at drawing temperature in a workingzone, and (d) drawing the product from the preform through an outlet inthe drawing chamber.
 2. A process as recited in claim 1, furthercomprising the steps of heating the fluid in a separate heating chamberand actively delivering the heated fluid to the drawing chamber.
 3. Aprocess as recited in claim 2, further comprising step of: after theheated fluid has flowed past the working zone, drawing the fluid back tothe heating chamber to be reheated.
 4. A process as recited in claim 3,further comprising the steps of:measuring the temperature of the heatedfluid at a chosen location at least of discrete time intervals, andusing the measured temperature values to adjust said fluid heating stepand the rate of said delivery step.
 5. A process as recited in claim 1,further comprising the step of: bringing a portion of the preform aboutto enter the drawing chamber to a pre-selected control temperature.
 6. Aprocess as recited in claim 1, further comprising the step of: shapingthe working zone by introducing an insulating means from the outlet ofthe drawing chamber a pre-selected distance toward the inlet of thechamber to form an effective outlet beyond which the drawn product isinsulated from the heated flow of fluid.
 7. A process as recited inclaim 1, wherein the preform is fed upward from below the drawingchamber and the product is drawn upward from above the drawing chamber.8. A process as recited in claiam 1, wherein the preform is a bundle ofdrawable elements.
 9. A process as recited in claim 8, furthercomprising the step of forming a preform having a cross-sectionaldimension relatively slightly larger than the desired final dimension ofthe product, and wherein the product is drawn just sufficiently to fusethe bundle into a voidless, integral mass.
 10. A process as recited inclaim 8, wherein the bundle is encased in a gas-tight envelope, theprocess further comprising the step of pulling a vacuum on the bundlewhile drawing the product.
 11. A process as recited in claim 10, furthercomprising the step of forming a preform having a cross-sectionaldimension relatively slightly larger than the desired final dimension ofthe product, and wherein the product is drawn just sufficiently to fusethe bundle into a voidless, integral mass.
 12. A process as recited inclaim 8, further comprising the step of:while drawing the product fromthe preform, twisting the bundle through a pre-selected angle.
 13. Anapparatus for drawing a preform of heat-softenable, drawable materialshaving a drawing temperature into a fiber or fiber-bundle product,comprising:(1) a furnace having,(a) a drawing chamber, (b) afluid-heating chamber containing gaseous fluid-heating means which meansare substantially completely radiatively shielded from the preformworking zone, (c) a forced convection element to deliver fluid from theheating chamber to the drawing chamber, (d) a drawing chamber inlet, (e)a drawing chamber outlet, (2) preform feeding mechanism adapted forfeeding the preform into said inlet, and (3) a product drawing mechanismadapted for drawing the product out of said outlet.
 14. Apparatus asrecited in claim 13, further comprising means of bringing the preform toa normalized temperature at the drawing chamber inlet.
 15. Apparatus asrecited in claim 14, wherein the means for bringing the preform to anormalized temperature include a fluid source connected by a conduit toa collar around the drawing chamber inlet, the collar having inwardlydirected apertures.
 16. Apparatus as recited in claim 13, furthercomprising an insulated sleeve extending from the drawing chamber outlettoward the drawing chamber inlet and having adjustable extension, thesleeve forming an effective outlet from the heated flow of fluid for thedrawn product.
 17. Apparatus as recited in claim 13, further comprisinga temperature measuring means located in the flow of heated fluid. 18.An apparatus as recited in claim 13, further comprising a centralcontrol mechanism functionally connected with the temperature measuringmeans and controlling the fluid heating means and the forced convectionelement and, through them, controlling the heat delivered by the flow ofheated fluid.
 19. Apparatus as recited in claim 18, wherein the centralcontrol mechanism is functionally connected to and controls theinsulated sleeve adjusting means.
 20. Apparatus as recited in claim 13,wherein the preform is formed of a bundle of elements and the apparatusincludes a gas-tight and evacuated envelope which encloses the preform.21. An apparatus as recited in claim 13, wherein the elements comprisingthe furnace are distributed symmetrically of a horizontal plane wherebythe preform may be fed and the product drawn from either the inlet orthe outlet respectively.
 22. An apparatus as recited in claim 16, havingan insulated sleeve extending both from the drawing chamber outlettoward the drawing chamber inlet and from the drawing chamber inlettoward the drawing chamber outlet.
 23. An apparatus as recited in claim13, in which the feeding mechanism, the furnace, and the drawingmechanism are mounted on gimbles, whereby the entire assembly may bepivoted in a vertical plane.
 24. A process for applying heat to aworkpiece longer than a treatment area of a furnace, the workpiecehaving a treatment temperature, comprising the steps of:(a) feeding theworkpiece continuously into an inlet in a treatment chamber, (b) heatinga fluid to a temperature at or above the treatment temperature of theworkpiece, using heating means which are shielded from the workpiece,(c) causing the heated fluid to flow past the portion of the workpiecein a treatment zone so that said portion of the workpiece is attreatment temperature, and (d) continuously drawing the treated portionof the workpiece through an outlet in the treatment chamber.